The high resolution attainable with transmission electron microscopy makes this an outstanding technique for examining the microstructure of materials. The direct examination of materials by transmission electron microscopy requires that the specimen to be examined is transparent to electrons. Consequently the thickness of the specimens must be restricted to 100 to 200 nm.
It has therefore been necessary to develop methods for preparing thin specimens of materials that have widely varying mechanical and chemical properties. Soft materials such as biological specimens may be prepared by microtoming, although difficulty is sometimes encountered when hard particles are present.
For some metals, semiconductors, and other inorganic materials, chemical etching and electrolytic techniques are suitable. In one widely used method the material to be thinned is placed in a jet etching tank and the etching process observed through a lens with a light source behind the specimen.
The etching action of the jet is maintained until perforation of the specimen occurs. Since the etching action is stronger at the centre of the jet than its periphery, perforation starts at the centre and spreads towards the periphery. Thus the etching process is immediately arrested when perforation occurs by flushing the specimen with an inhibiting wash, leaving adjacent areas around the perforation which are usually sufficiently thin to allow micrographs to be taken during examination under the electron microscope.
For materials for which suitable chemical etchants do not exist, such as some glasses, ceramics, and geological specimens, various mechanical preparation techniques have been tried. The specimens may be crushed and fine slivers selected, or thin sections may be produced by very careful mechanical polishing. These operations require considerable skill and can generally not be applied to brittle granular materials with voids.
A large variety of materials which do not lend themselves to chemical treatment may be thinned by ion erosion.
In typical known thinning equipment ion beams of about 2 mm diameter from two sources impinge centrally on either side of a specimen. See Franks U.S. Pat. No. 4,128,765. A hole or perforation is allowed to form in the specimen by the ion beam, which acts in a similar way to the chemical jet during chemical etching, when the ion beam is immediately turned off to leave adjacent thin areas around the perforation transparent to electrons. Such method and apparatus is shown in Franks U.S. Pat. No. 4,340,815.
Ion erosion has proved an increasingly valuable tool to electron microscopists especially those engaged in examining classes of material such as ceramics, impurity-doped semiconductors and alloys.
Difficulty however arises when dealing with ionic and covalent materials since bombardment with the charged particles of the ion beam may deleteriously affect the structure of the material.